A scale is said to have constant structure (CS) if its generic interval classes are distinct. That is, each interval occurs always subtended by the same number of steps. This means that you never get something like an interval being counted as a fourth one place, and a fifth another place.

The term "constant structure" was coined by Erv Wilson. In academic music theory, constant structure is called the partitioning property, but Erv got there first.

To determine if a scale is CS, all possible intervals between scale steps must be evaluated. An easy way to do this is with an interval matrix (Scala can do this for you). A CS scale will never have the same interval appear in multiple columns of the matrix (columns correspond to generic interval classes).
Examples

This common pentatonic scale is a constant structure: 1/1 - 9/8 - 5/4 - 3/2 - 5/3 - 2/1
Here is the interval matrix of this scale:

	1	2	3	4	5	(6)
1/1	1/1	9/8	5/4	3/2	5/3	2/1
9/8	1/1	10/9	4/3	40/27	16/9	2/1
5/4	1/1	6/5	4/3	8/5	9/5	2/1
3/2	1/1	10/9	4/3	3/2	5/3	2/1
5/3	1/1	6/5	27/20	3/2	9/5	2/1

Note that every interval always appears in the same position (column). For example, 3/2, which happens to appear three times, is always the "fourth" of this scale - never the "third" or "fifth".


This pentatonic scale is not a constant structure: 1/1 - 25/24 - 6/5 - 3/2 - 5/3 - 2/1
Its interval matrix:

	1	2	3	4	5	(6)
1/1	1/1	25/24	6/5	3/2	5/3	2/1
25/24	1/1	144/125	36/25	8/5	48/25	2/1
6/5	1/1	5/4	25/18	5/3	125/72	2/1
3/2	1/1	10/9	4/3	25/18	8/5	2/1
5/3	1/1	6/5	5/4	36/25	9/5	2/1

Note the highlighted intervals that occur in more than one column. For example, 5/4 may occur as both the "second" and "third" steps of the scale. Thus, this scale does not have constant structure.


Another example of a familiar scale that is not CS is the 7-note diatonic scale in 12edo.
Interval matrix as steps of 12edo:

	1	2	3	4	5	6	7	(8)
0	0	2	4	5	7	9	11	12
2	0	2	3	5	7	9	10	12
4	0	1	3	5	7	8	10	12
7	0	2	4	6	7	9	11	12
9	0	2	4	5	7	9	10	12
11	0	2	3	5	7	8	10	12
12	0	1	3	5	6	8	10	12

Interval matrix as note names:

	1	2	3	4	5	6	7	(8)
C	C	D	E	F	G	A	B	C
D	C	D	Eb	F	G	A	Bb	C
E	C	Db	Eb	F	G	Ab	Bb	C
F	C	D	E	F#	G	A	B	C
G	C	D	E	F	G	A	Bb	C
A	C	D	Eb	F	G	Ab	Bb	C
B	C	Db	Eb	F	Gb	Ab	Bb	C

F# and Gb are the same pitch (600 cents) in 12edo, and this interval occurs as both an (augmented) fourth and a (diminished) fifth - so not constant structure. (However, a meantone tuning of this scale, in which F# and Gb are distinguished, would have constant structure.)

Density of CS Scales in EDO's

EDO	Number of CS Scales	Percent of Scales CS	Corresponding Fraction
1	1	100.0%	1/1
2	1	100.0%	1/1
3	2	100.0%	1/1
4	2	66.7%	2/3
5	5	83.3%	5/6
6	4	44.4%	4/9
7	11	61.1%	11/18
8	11	36.7%	11/30
9	22	39.3%	11/28
10	20	20.2%	20/99
11	45	24.2%	15/62
12	47	14.0%	47/335
13	85	13.5%	17/126
14	88	7.6%	88/1161
15	163	7.5%	163/2182
16	165	4.0%	11/272
17	294	3.8%	49/1285
18	313	2.2%	313/14532
19	534	1.9%	89/4599
20	541	1.0%	541/52377

See also

Gallery of CS Scales
Scale properties simplified
epimorphic
Constant structure (Tonalsoft Encyclopedia)
Introduction to Erv Wilson's Moments of Symmetry